1. Field of the Invention
This invention relates to the prevention of outer skin puncture of thin skin sandwich laminates by direct lightning attachment.
This invention relates to lightning protection systems and, more particularly, to such a system for protecting electrically conductive composite material aircraft structures from puncture that have thin outer skin structure or thin skin sandwich structure. The protection method is to place a controlled electrically conductive layer outwardly of the conductive skin layer and in electrical contact with the conductive skin so as to disperse the current from severe lightning direct attachment, and that has means for preventing puncture of the outer skin surface and minimize damage to the structure.
2. Background Art
Conventional aluminum aircraft structures typically have inherent lightning protection. An aluminum skin structure has uniform and predictable material properties, including the electrical properties. Thus, protection against skin puncture by lightning can be provided simply by sizing the aluminum skin thickness to a range of 0.404 to 0.060 inch. In areas of conventional aircraft structure where the skin is electrically non-conductive, such as radomes and aerodynamic fairings, metal bus bars can be applied to the exterior surface to direct the lightning currents to the aluminum structure.
Providing lightning protection for electrically conductive composite material structure, such as carbon fiber reinforced plastic, is much more difficult and complicated than for typical aluminum structure. Conductive composites are nonhomogeneous and are considerably less conductive than aluminum. In thick skin structure, i.e., greater than 0.10 inch, the skin may be thick enough to resist puncture but the structure is still subject to severe skin surface damage, which can be minimized by applying relatively heavy metal or insulating coatings, or a combination in layers, to provide adequate protection as noted in previous patents that deal with thick skin electrically conductive composite structure.
Intuitively, one would expect to use heavier metal meshes to protect thin skin electrically conductive composite structure. However, for electrically conductive thin skin and sandwich composite structure, the structure is easily punctured by lightning attachment even when the heavy metal mesh coatings are used. Using heavier weight coverings comparable to aluminum skin thickness would provide adequate protection but would negate the weight saving advantages of composite structure over that of aluminum.
It is the object of this invention to provide a lightweight single layer protection system that will provide surface protection and prevent puncture of thin skin electrically conductive composite aircraft structure. Previously the primary considerations for the selection of the lightning protection material has been based more upon the mechanical, corrosion and weight aspects of the material.
It is further the object of this invention to select the material based primarily on the electrical properties of the protection layer wherein a narrow window of an electrical property, that is surface conductivity, provides optimum protection for thin skin electrically conductive structure. It is fortuitous that this leads to a lighter weight solution for thin skin conductive composite structure than the current methods used to protect thick skin composite structure. By applying a lighter weight metal mesh that has a controlled electrical surface conductivity which is less than that used for thick skin composite structure, skin puncture from severe lightning strike attachment can be prevented for thin skin electrically conductive composite structure.
The lighter weight conductive coatings would also provide protection for thin skin structure that is non-electrically conductive, with or without electrically conductive core structure.
Critical features of the present invention include: 1) The selection of the mesh weight based upon the electrical surface conductivity to provide a precisely controlled and efficient protection to prevent puncture of thin skin structure; 2) The heavier more conductive mesh weights do not necessarily provide better lightning protection than the lightweight meshes; 3) For carbon fiber reinforced plastic skins, the electrically conductive layer makes electrical contact with the electrically conductive carbon fiber skin structure to make electrical bonding inherent in the structural makeup.
Another important feature is that the metal mesh layer also provides Electromagnetic Interface (EMI) shielding and when combined with the inner surface foil EMI shielding, a reduction in thickness of the inner surface foil can be made to minimize wrinkling.